1. Field of the Invention
The present invention relates to a conductive roller to be used for an image-forming apparatus such as a printer, a copying machine, a facsimile, an ATM, and the like.
2. Description of the Related Art
In a large number of image-forming apparatuses adopting an electrophotographic method, a contact charge method and a contact transfer method are adopted in which a very small amount of ozone is generated. A roller-shaped member excellent in its wear resistance and toner-transporting performance is used in most of image-forming apparatuses. The roller-shaped member is constructed of a columnar core made of SUS or iron and a conductive layer, disposed on the peripheral surface of the core, which is adjusted to 1×105Ω to 1×1010Ω in its electric resistance by means of carbon or an ionic conductive agent.
A part of the conductive layer is composed of an elastic material so that a nip can be securely formed for other members such as a photoreceptor and an intermediate transfer member both carrying images. A foam can be used as the conductive layer.
The conventional image-forming apparatus in which the conductive roller is used has problems described below.
Because pulverized toner having a comparatively large diameter has been widely used, toner which has not been transferred can be collected by a cleaning roller or the like. But polymeric toner which has been used in recent years is a micro-sphere having a diameter of several microns close to a perfect sphere. Therefore it is difficult to collect it. Such being the case, there is a demand for the development of a conductive roller excellent in its transfer efficiency to eliminate the need for cleaning toner.
Rubber forming the conductive layer contains various components including the residue of an initiator used in synthesizing a rubber component, by-products generated when the rubber component is synthesized, low-molecular-weight substances contained in the rubber component, a vulcanizing agent, a softener, and a plasticizer added to the rubber component in molding the rubber composition into a rubber roller. Many of these components react with substances composing the photoreceptor and the intermediate transfer member. When the conductive roller is left for a long time with the conductive roller being pressed against the photoreceptor and the intermediate transfer member, the components of the conductive roller exude therefrom and stick to the photoreceptor and the intermediate transfer member, thus forming a defective image. In a serious case, the surface of the photoreceptor and that of the intermediate transfer member react with the components of the conductive roller and are modified and whitened. Consequently subsequently formed images are defective.
To solve this problem, it is conceivable to coat the surface of a single-layer conductive roller with a substance forming a barrier layer for preventing components contained in the rubber from exuding therefrom. But the conductive roller has a plurality of layers. Thus the material cost is high and in addition the production step increases. Therefore the cost for producing the conductive roller is high.
To solve these problems, the present applicant has developed the following conductive rollers as disclosed in the following patent documents 1 and 2.
The conductive roller disclosed in Japanese Patent Application Laid-Open No. 2006-259131 (patent document 1) has the conductive foam consisting of the rubber composition containing the specific acrylonitrile butadiene rubber and the specific epichlorohydrin rubber at the predetermined ratio. The conductive foam has a maximum cell diameter not more than 200 um, a minimum cell diameter not less than 10 um, and a Shore E hardness of 20 to 50.
The conductive roller has micro-cells and a low hardness, and a sufficient nip width is obtained. Consequently the conductive roller has an improved transfer efficiency, and there is a decrease in the amount of residual toner which remains on the surface of the photoreceptor and the intermediate transfer member without being transferred. Thus this conductive roller has the advantage that the conductive roller is capable of complying with the recent demand for the development of the conductive roller which eliminates the need for cleaning toner.
But it has been found that in the conductive roller disclosed in the patent document 1, there is room for improvement in the pollution of other members in contact therewith.
In the conductive roller disclosed in Japanese Patent Application Laid-Open No. 2006-145636 (patent document 2), the oxide film is formed on the outermost surface of the conductive elastic layer. The predetermined relationship is established between the initial electric resistance R0 of the conductive roller when a voltage of 1000V is applied thereto and the electric resistance R100 thereof after the voltage is continuously applied thereto for 100 hours. In the third embodiment of the conductive roller, the conductive elastic layer is composed of the continuous phase consisting of the acrylonitrile butadiene rubber (NBR), the first uncontinuous phase consisting of the epichlorohydrin rubber, and the second uncontinuous phase consisting of the ethylene-propylene-diene copolymer rubber (EPDM). It is essential that the ionic conductive agent is locally present in the first uncontinuous phase consisting of the epichlorohydrin rubber.
The conductive roller disclosed in the patent document 2 has the advantage that it is capable of effectively preventing other members from being polluted and can be supplied at a low cost in a large amount without coating the surface of the conductive elastic layer with an expensive material forming the barrier layer.
But in the conductive roller disclosed in the patent document 2, the cell diameter and the number of cells when a foam is used as the conductive elastic layer are not investigated. The cell diameter and the number of cells affect the hardness and electric resistance of the conductive roller. Thus there is room for improvement in producing a conductive foam having a proper number of cells and a proper cell diameter.
Patent document 1: Japanese Patent Application Laid-Open No. 2006-259131
Patent document 2: Japanese Patent Application Laid-Open No. 2006-145636